1. Field of the Invention
The present invention relates to a generator of periodic signals having any period and a given shape. The generator may be applied to any field, including the field of television.
2. Discussion of the Related Art
The related art and the invention shall be described in the context of the processing of television signals to facilitate the description. However, the principles of the invention have broader application and the description should not in any way limit the scope of the invention.
Horizontal scanning signals as well as vertical scanning signals are known in the field of television signals and are used to display images on cathode-ray tube screens. The horizontal scanning signal is a signal applied to deflection coils or electrodes of one or more electron guns sending out electrons from a cathode to an electroluminescent screen. To display an image, the screen is scanned in order to display a certain number of lines in vertical relation to one another from the top to the bottom of the screen.
The number of lines displayed on a television screen varies according to the resolution desired, as well as according to the display standards used in the different regions of the world. While 420-line or 625-line images are currently known, new standards stipulate 1024 lines and even 2048 lines. Furthermore, the frequency of refreshing of the images on the screen, which is currently 25 images per second, also depends on the qualities and standards desired. For example, a rate of 50 images per second is envisaged to prevent the effects of scintillation.
These different standards and different requirements currently require scanning signal generators that are specific to each of the uses.
Furthermore, a cathode-ray tube screen depicts the image in distorted form because of so-called "pincushion distortion." For horizontal scanning, pincushion distortion refers to the effect wherein the length scanned by the trace of the electron gun on the screen, namely the spot, depends on the altitude of the line to be scanned. In practice, the length traveled by the spot for the lower and upper lines is greater than it is for the median lines. The variation is actually continuous so that, at its left-hand and right-hand edges, the image has parabola-shaped profiles.
This phenomenon is due to the application and to the deflection devices of the electron guns. This phenomenon is already known.
Attempts have been made to correct this problem by applying additional voltage or current to these deflection devices as a function of the line to be corrected so that the field of exploration is always the same, whatever the altitude of the line. For example, a circuit known as "TDA 815" by S.G.S. THOMSON MICROELECTRONICS, is capable of producing such correction signals. The implementation of a circuit of this type necessitates, however, prior setting, first as a function of the standard according to which the monitor will work and, second, as a function of deterioration of the particular display monitor. This results in a generation of instruments in which the mode of use and the standards must be planned in advance.